# Muscles require energy for contraction. Explain how the body meets the energy demands of skeletal muscle during vigorous exercise, including what happens when oxygen supply becomes insufficient.

> Pearson Edexcel A-Level Biology A (9BI0) — 4.2 Muscle contraction · Explain · 5 marks

> During a 400-metre sprint, a runner's muscles contract repeatedly and with great force. The cardiovascular system attempts to deliver more oxygen to the working muscles, but demand can outstrip supply.

## Mark scheme (5 marks)

1. Muscles need ATP / energy for contraction
2. Aerobic respiration occurs / glucose and oxygen are used to release energy (ATP) in mitochondria
3. During vigorous exercise, oxygen cannot be delivered fast enough to meet demand / muscles respire anaerobically
4. Anaerobic respiration produces lactic acid (in animals/humans)
5. Lactic acid causes muscle fatigue / cramp / pain, and an oxygen debt is created which must be repaid after exercise to oxidise the lactic acid

## Related

- [Revision notes for Pearson Edexcel A-Level Biology A (9BI0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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